1. Field of the Invention
The present invention relates to laser sources used in optical communication systems and, more particularly, to an optical fiber laser source whose repetition rate is precisely controlled by an accurate frequency signal standard.
2. Description of the Related Art
Laser sources, such as optical fiber lasers, have a need for producing high-repetition pulses in the range of 10 Giga (G) bits/second, a pulse duration of less than 2 picoseconds, essentially no pulse drop-out, and low phase and amplitude noise. The optical fiber lasers commonly employ passive or active mode locking to attain these ends.
Passive mode locking relies on incorporating elements in a fiber laser that transmit high-intensity light more easily than low-intensity light. Since a train of pulses has a higher peak power than a continuous beam, such a laser will produce very brief pulses. A traditional means of passive mode locking is the use of a fast saturable absorber. In a typical saturable absorber a light beam encounters a finite number of absorber molecules. When all of the molecules are excited, the dye is bleached and the dye becomes transparent to the light. Saturable absorbers with a fast recovery time absorb long, low-intensity pulses but bleach out with brief, high-intensity pulses. Passive mode locking depends on the laser having an overall lower loss for higher-energy pulses than for lower-energy pulses, but one way to produce high-energy pulses is for one pulse to steal energy from another. For this reason, passive mode-locked lasers have an inherent tendency to produce incomplete pulse streams.
For certain applications of the optical communication systems, it is desired that the operations being performed by is different users be synchronized to a standard frequency source. In such synchronized optical communication systems, a lasing material serving as a laser source that provides coherent light is termed as being "actively mode-locked," meaning that its repetition rate can be controlled by an accurate external electronic standard, and such systems are described in a first technical article by T. F. Carruthers, I. N. Duling, III, and M. L. Dennis, "Active-Passive Mode Locking in a Single-Polarization Erbium Fiber Laser," published in Electron Lett. 13, (1994), and in a second technical article of T. F. Carruthers and I. N. Duling, III, "A 10-GHz, Single-Polarization, Actively-Mode-Locked Picosecond Erbium Fiber Laser," Optical Fiber Conference, vol. 2, 1996 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1996) pp. 7-8, both of which technical articles are herein incorporated by reference for all purposes. Moreover, mode-locked laser light sources are described in U.S. Pat. Nos. 4,665,524 ('524); 5,546,414 ('414); and 5,574,739 ('739) all of which are herein incorporated by reference.
Active mode locking alone produces an uninterrupted string of pulses, but the pulse durations are governed by the Kuizenga-Siegman relationship more fully disclosed in the technical article entitled, "FM and AM Mode Locking of the Homogeneous Laser--Part I: Theory" of D. J. Kuizenga and A. E. Siegman, IEEE J. Quantum Electron. 6,694 (1970), which is herein incorporated by reference. During such active mode locking, the pulse duration tends to be lengthened by the gain bandwidth .LAMBDA..sub.g =1/.tau..sub.g of the laser, but the time window T.sub.a of the active mode locking has a pulse-shortening influence. The actual pulse duration .tau. turns out to be proportional to the geometric mean of the two influences: .tau..congruent.(.tau..sub.g .multidot.T.sub.a).sup.1/2. The pulse duration that is typically produced by active mode locking at a 10 Giga Hertz (GHz) repetition rate is a minimum of about 5 picoseconds, too long for many intended applications, and it is desired to further shorten the pulse.
Short pulse durations may be attained in an optical fiber laser through a process called soliton pulse shortening. A pulse propagating in an optical fiber will, under certain very general conditions, tend to shape itself into a specific type of pulse called a soliton--a "solitary wave"--that propagates without changing its shape. Such a pulse has a specific intensity profile, and, other things being equal, a higher-energy pulse will reshape itself into a briefer soliton than will a lower-energy pulse. Soliton pulse compression provides an additional pulse-shortening mechanism in a manner more fully disclosed in the technical article entitled, "Solitary-Pulse Stabilization and Shortening in Actively Mode-Locked Lasers" of F. X. Kartner, D. Kopf, and U. Keller, J. Opt. Soc. Am. B 12,486 (1995), which is herein incorporated by reference.
Further details for producing short duration pulses are disclosed in the technical article of D. J. Jones, H. A. Haus, and E. P. Ippen, "Subpicosecond Solitons In An Actively Mode-Locked Fiber Laser," Opt. Lett. 21, 1818 (1996), which is herein incorporated by reference. It is desired that the pulses produced by an optical fiber laser be further improved, especially their duration being further shortened or reduced.
The actively mode-locked laser sources may be further improved if their insensitivity from environmental error contributors, such as environmentally-induced birefringence variations, is increased. Fiber birefringence can be a major problem, since the polarization state of a pulse can become scrambled in as little as a few cm of propagation. Birefringence can be due to residual stresses in the fiber from its drawing, or to stress induced from winding the fiber. Birefringence can also change with temperature and other environmental factors, causing time-varying polarization states. A special fiber, called polarization-maintaining fiber, has an intrinsic birefringence larger than any environmental birefringence it will encounter, so that light launched with its polarization along a primary axis will remain on that axis. The environmental error contributors degrade the stability of the laser source in a manner more fully described in the already incorporated by reference '524 patent. Increased insensitivity is accomplished by polarization maintaining (PM) or by birefringence-compensation techniques, both known in the art and more fully described in the previously incorporated by reference technical articles. Further, polarization-maintaining means, such as polarization-maintaining fiber, is more fully described in the already incorporated by reference '739 patent.
In addition to the above desired features, the optical fiber laser source satisfies a wide range of operating requirements if it provides pulses having low timing error and low amplitude jitter, as well as having a low pulse drop-out rate, that is, missing pulses in the associated pulse train produced by the laser source that are low in number, more particularly, less than one in 10.sup.12.
It is desired that an optical fiber laser source be provided that is actively mode-locked, insensitive to environmentally induced birefringence variations, and generates a pulse train in the GHz range that has a pulse drop-out rate less than 10.sup.-12, wherein each pulse has a low timing error and a low amplitude jitter.